1. Technical Field
This invention relates to a light amplification optical fiber utilized mainly for an optical communication system; and a light amplifier using the optical fiber.
2. Background Art
In recent years, a demand for communication by internet, etc. has suddenly increased, and an increase in the transmission capacity of an optical transmission passage is being actively discussed. In accordance with these facts, an increase in the capacity of a wavelength division multiplexing (WDM) optical transmission system, i.e. an increase in the number of channels and the expansion of a transmission band are progressing. In order to attain an increase in the capacity of a WDM optical transmission system, it is indispensable to increase the level of an output from a signal light amplifying light amplifier and attain the expansion of a transmission band, and a light amplifier as an important key device has increasingly attracted the technicians"" attention.
With an increase in the number of channels for a WDM signal, total signal light input power increases, and higher saturation power has been demanded with respect to a light amplifier which is adapted to collectively increase the number of channels and signal light input power. Under the circumstances, the increasing of a light amplifier output has been forwarded by increasing the efficiency of a light amplification optical fiber and a pumping source output.
The discussing of the expansion of a gain band for a light amplifier is also being forwarded so as to expand a transmission band, and a light amplifier having a gain band of a wavelength larger than that a gain band in a related art light amplifier has been attained in practice by using an elongated light amplification optical fiber.
However, the above-mentioned increase in an output from and the length of a light amplification optical fiber has caused a new problem to arise, i.e., nonlinear phenomena which had not been observed in a related art light amplification optical fiber have occurred. The main nonlinear phenomena include four-wave mixing and cross phase modulation. Since the transmission quality of signal light is deteriorated due to these nonlinear effects, the suppression of the occurrence of nonlinearity has become necessary and indispensable.
The waveform distortion due to nonlinear phenomena can be estimated by the following equation (1).
"PHgr"NL=(2xcfx80/xcex)xc2x7(n2/Aeff)LeffPxe2x80x83xe2x80x83(1)
In this equation (1), Leff represents an effective length of an amplification optical fiber, Aeff an effective core area n2 a nonlinear refractive index, xcex a signal light wavelength, and P input light intensity.
It is understood from the equation (1) that, in order to suppress the occurrence of waveform distortion "PHgr"NL, it is preferable to (1) reduce the effective length of a fiber, and (2) minimize a nonlinear constant (n2/Aeff).
In order to reduce a length of a fiber used, it is necessary to heighten a gain coefficient per unit length by increasing an amount of addition of a rare earth element. However, when the concentration of erbium is increased in an erbium-doped optical fiber, which is being put to practical use as a light amplifier for 1.55 xcexcm band, a decrease in the amplification efficiency due to concentration quenching occurs.
In general, the concentration of erbium at which concentration quenching starts occurring is several hundred wtppm in the case of a pure quartz (SiO2) host, and it is said to be around 1000 wtppm even in the case of Al2O3xe2x80x94SiO2 host to which aluminum having a concentration quenching suppressing effect is co-doped (refer to, for example, R.I.LAMING, D.N.PAYNE, F.MELI, G.GRASSO, E.J.TARBOX, xe2x80x9cSATURATED ERBIUM-DOPED FIBRE AMPLIFIERSxe2x80x9d, OAA ""90.Tech.Digest MB3.). Therefore, there is a limit to the reduction of the length of an erbium-doped optical fiber.
The increasing of an effective core area Aeff may be mentioned as another nonlinearity suppressing method. In general, an effective area of an erbium-doped optical fiber has a value one digit smaller than that of a regular single-mode optical fiber (SMF). The reason for the above is to form excellent inverted population over the whole of a core by increasing a relative refractive index difference xcex94 of a core portion with respect to a clad portion, and thereby heightening the pumping light density so as to improve the gain characteristics of the erbium-doped optical fiber with a high efficiency.
Furthermore, a light amplification optical fiber contains various kinds of large amounts of dopants in a core portion thereof as compared with a regular transmission optical fiber, and this makes it more difficult to control an effective core area.
A light amplification fiber contains many elements, for example, an element for improving the solubility of a rare earth element for melting the rare earth element in host glass, an element for increasing a refractive index of host glass, an element for varying gain wavelength characteristics thereof, an element for improving the amplification characteristics thereof, an element doped to regulate the viscosity thereof and the like besides rare earth elements constituting active substances. These elements, which are other than the rare earth elements, doped to a core portion are called co-dopants.
The addition concentration of these co-dopants is determined from the viewpoint of the optimization of the amplification characteristics of the light amplifier. Conjointly with the diversity of the co-dopants, the above matter makes it very difficult to control the refractive index.
The germanium has heretofore been used as a co-dopant for improving the refractive index of a core portion.
The germanium is effective not only in improving the refractive index but also in regulating the viscosity of a core portion. Such co-dopants as are mentioned above which are doped to a core portion of a light amplification optical fiber are all substances for improving the refractive index just as the germanium.
However, since the relative refractive index difference xcex94 becomes high due to the addition of the co-dopants, it is difficult to suppress the nonlinearity of a light amplification optical fiber. It is necessary as previously mentioned to reduce the using length, i.e. effective length of a light amplification optical fiber for the purpose of suppressing the nonlinearity thereof. However, in order to meet this requirement, it is necessary to improve the gain coefficient per unit length. Therefore, the addition amounts of a rare earth element and a co-dopant have to be increased, and this causes the relative refractive index difference xcex94 to further increase.
In view of these problems, a method of improving a limit level of addition concentration of erbium, and a new method of attaining a desired effective core area Aeff without depending upon the addition concentration of co-dopants, such as a rare earth element, etc. for practically obtaining a nonlinear phenomena-suppressed amplification optical fiber have been demanded.
The present invention has been made in view of the above-mentioned circumstances, and provides a nonlinear phenomena-suppressed light amplification optical fiber, and an optical amplifier using the fiber.
According to an aspect of the present invention, the light amplification optical fiber has the following construction, i.e. this optical fiber has a core portion, and not less than one layer of clad portion surrounding an outer circumference of the core portion, erbium ions being doped to the core portion, at least one kind of rare earth element ions which are other than the erbium ions, and which have ion radius of not lower than 70% and not higher than 130% of that of the erbium ions, being further doped to the core portion.
According to another aspect of the present invention, the light amplification optical fiber has aluminum doped to the core portion, the number of the ions of the aluminum doped to the core portion being not less than 6 times as large as a total ion number of the rare earth element contained in the core portion.
According to still another aspect of the present invention, the light amplification optical fiber has phosphorus doped to the core portion.
According to a further aspect of the present invention, the light amplification optical fiber has a substance reducing a refractive index of pure quartz, this substance being doped to the core portion, the substance reducing the refractive index of pure quartz being, for example, fluorine.
According to another aspect of the present invention, the light amplification optical fiber has germanium doped to the core portion in some cases, or does not substantially have germanium in the core portion in some cases.
According to still another aspect of the present invention, the light amplification optical fiber has in at least a part of the clad portion a substance increasing a refractive index of pure quartz, this substance which increases a refractive index of pure quartz being at least one of, for example, germanium, phosphorus and aluminum.
According to a further aspect of the present invention, the light amplification optical fiber has a concentration of the erbium doped to the core portion of as high as over 1000 wtppm, the concentration being allowed to reach as high as 2000 wtppm level.
According to another aspect of the present invention, the light amplification optical fiber has a rare earth element, which is other than erbium, doped to the core portion, the rare earth element being an element selected from the group of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb and Lu.
According to still another aspect of the present invention, the light amplification optical fiber has an example of refractive index distribution constitution thereof, in which a relative refractive index difference of the core portion with respect to the clad layer on the immediately outer side of the core portion is smaller than 1.0%.
According to a further aspect of the present invention, the light amplifier has the above-mentioned light amplification optical fibers used as light amplification optical fibers.
According to the present invention described above, the increasing of the concentration of erbium to a high level can be attained without causing a decrease in the efficiency ascribed to concentration quenching to occur, and the length of a fiber in use can be reduced. The reason for the above resides in that ions of a rare earth element, which is other than erbium, co-doped surround those of erbium to cause the distances among the erbium ions to increase, and the interaction of the erbium ions to be suppressed.
The addition of a substance reducing the refractive index of the core portion and the non-addition of germanium thereto enable a light amplification optical fiber in which a nonlinearity constant is further reduced to be obtained in practice. The reason for the above resides in that the degree of freedom of regulating a refractive index difference between the core and clad increases not depending upon the kind and concentration of dopants, such as a rare earth element, aluminum, phosphorus, etc., this enabling the effective core area (Aeff) to be increased arbitrarily, and the nonlinearity constant (n2/Aeff) to be increased in consequence.
The amount of waveform distortion due to nonlinear phenomena is proportional to a product of an effective length Leff and a nonlinearity constant (n2/Aeff). According to the present invention, the effective length and nonlinearity constant can be reduced respectively. As a result, the occurrence of nonlinear phenomena in the light amplification optical fiber can be suppressed, and a light amplifier free from the deterioration of the transmission quality of an optical signal can be obtained in practice.